Nowadays, fuel cells which are excellent in power generation efficiency and which do not emit CO2 are being developed from the viewpoint of global environment conservation. Fuel cells are classified into types such as a phosphoric-acid fuel cell, a solid oxide fuel cell and a proton-exchange membrane fuel cell on the basis of kind of electrolyte which is used for the fuel cell. Among these fuel cells, proton-exchange membrane fuel cells are applied to stationary type electric generators for household use, onboard power supplies of fuel-cell electric vehicles and so forth, because proton-exchange membrane fuel cells can work at a low temperature of 100° C. or lower, can be started up in a short time and can be suitably reduced in size.
In the case of proton-exchange membrane fuel cells, desired power is obtained by connecting many cells, which consist of proton-exchange membranes being placed between separators, in series. Graphite has been used for separators up to now, because good conductivity and corrosion resistance under a condition of high potential are required for the separators. However, there is a problem in that graphite has a low impact resistance and graphite is not easy to manufacture flow channels of hydrogen and so forth. Therefore, the application of stainless steel, which has high impact resistance and workability, to the separators is being investigated.
However, it is difficult to use stainless steel as a separator as it is, because stainless steel has high contact resistance due to a passivation film which is formed on the surface of the stainless steel.
In order to solve the problem, focusing on a passivation film of stainless steel, methods for decreasing contact resistance by reforming the passivation film are being investigated (for example, patent documents 1 through 3).
Patent document 1 discloses stainless steel to be used for a separator of a proton-exchange membrane fuel cell having a chemical composition containing, by mass %, C≦0.03%, N≦0.03%, 20%≦Cr≦45% and 0.1%≦Mo≦5.0%, the atomic ratio of Cr to Fe Cr/Fe in a passivation film being 1 or more.
Patent document 2 discloses a separator for a proton-exchange membrane fuel cell, which is made of stainless steel, having a chemical composition containing, by mass %, 15%≦Cr≦40% and 1%≦Mo≦5%, and Mo/(Mo+Cr+Fe) in terms of atomic ratios comprising Mo, Cr and Fe in a passivation film being 0.3 or less and 1.5 times or more the value of Mo/(Mo+Cr+Fe) in a substrate.
Patent document 3 discloses a separator for a proton-exchange membrane fuel cell having a chemical composition containing, by mass %, 16%≦Cr≦40% and 1%≦Mo≦5%, micro pits whose diameters are 0.01 μm or more and 1 μm or less being formed in the entire region of the surface and the atomic ratio of Cr to Fe Cr/Fe in a passivation film being 4 or more.